1. Field of the Invention
The subject invention relates to lasers and, more particularly, to an improved Q-switch for use in a laser resonant cavity.
2. Description of Related Art
The quality factor, Q, of a laser resonant cavity is defined as the ratio of energy stored in the cavity to the energy loss per cycle. The higher the Q, the lower the losses. In the case of a method known as Q-switching, the losses in the cavity are initially kept high, i.e., low Q, for a period of time during which the laser gain medium is pumped up to a very high population inversion. The Q is then rapidly restored to a high value, i.e., low losses, causing a short pulse of laser light with high peak power (a so-called "giant" pulse) to be generated.
Q-switching may be accomplished "actively" or "passively." Passive Q-switching may be accomplished by the insertion of a saturable absorber into the cavity, i.e., a material which has relatively high losses (absorption) at low incident intensity at the laser wavelength, but possesses significantly lower losses at high incident intensity. The saturable absorber therefore results in a low cavity Q during the initial pumping of the gain medium; however, when the gain is sufficient to overcome the low intensity losses and lasing begins, the absorber is quickly "bleached" to a low-loss state (high Q) and a giant pulse is generated.
The passive Q-switch used for 1.06 .mu.m lasers has frequently consisted of a BDN dye-impregnated acetate sheet (manufactured by Kodak), sandwiched between two glass plates with an index-matching adhesive. The BDN dye used in this case has a large absorption cross-section (.about.10.sup.-16 cm.sup.2), but suffers from considerable excited-state absorption (ESA). The contrast C (ratio of ground-state to excited-state cross-section) is about 5. The maximum transmittance (completely saturated) is given by: ##EQU1## where T.sub.0 is the small-signal transmittance. For a dye Q-switch with a small-signal transmittance of 40%, T.sub.max =83%.
An alternative saturable absorber material for 1 .mu.m radiation utilizes the tetravalent chromium ion, e.g. Cr.sup.4+ :YAG. A contrast of C.apprxeq.10 has been measured for Cr.sup.4+ :YAG. This means that a Cr.sup.4+ :YAG Q-switch having T.sub.0 =40% can be bleached to T.sub.max =91%. A higher T.sub.max results in fewer losses during the Q-switched laser pulse and a higher overall laser efficiency. The saturable absorber material Cr.sup.4+ :YAG has a significantly smaller cross-section (.about.2.times.10.sup.-18 cm.sup.2) than the BDN dye, as well as a much longer relaxation lifetime: 4 .mu.s for Cr.sup.4+ :YAG, compared to 5 ns for the BDN dye, both times measured at room temperature.
One problem that has been observed with the Cr.sup.4+ :YAG absorber material is a direct result of the longer lifetime. Because a bleached portion of the Cr.sup.4+ Q-switch remains open for a few microseconds, additional pulses have been observed following the primary pulse with delays of a few hundred nanoseconds or so. This characteristic is highly undesirable for many applications.